The present invention relates to systems and methods for treating landfill waste using a portable pugmill system.
Operating landfills are provided with liquid and solid waste on a constant basis. Some of these waste materials must be further processed before they can be legally entombed within a landfill. This processing may include mixing waste liquids, sludges and other semi-solid wastes with absorbent materials or other solid (dry) wastes (termed here “waste solidification operations”), before the resulting mixture is sufficiently devoid of any “free liquids” so that it may be legally disposed within an active landfill cell. Examples of liquid wastes that may be treated by the present invention include, but are not limited to, various approved liquid wastes, landfill leachate, or surface water from the landfill or off-site locations.
Currently, the required waste solidification operations may be performed in “batch” processing techniques using open-top, leakproof containers located in an area of the landfill that has both a certified liner and an operating leachate collection system. This is a time-consuming and inefficient process which requires a large portion of the landfill that could otherwise be devoted to stored waste.
Another processing technique to achieve sufficient waste solidification that has been used in the past has been a large, expensive fixed system, using a processing plant with a fixed pugmill fed by a fixed, large overhead silo not adapted to be portable. No discharge conveyor is used; instead, a truck drives under the pugmill, which drops the solidified waste into the truck, for delivery to the landfill.
Pugmills used in the systems of the present invention can weigh 6 tons or more. Horsepower requirements for the required multiple motors (for, e.g., pugmill, conveyors, generators, etc.) may be 300-350 hp or more. Also, the area of a landfill typically comprises low-grade, unstable materials such as wet clay with little bearing strength, and spongy municipal solid waste (residential garbage). Further, one end of each of the conveyors is typically ten feet or more off the ground. These issues, including issues of alignment, vibration control, leveling, stability in wind, deterioration in rain and snow, adequate 3-phase electrical power source, and practicality of maintenance, which can routinely be handled by a fixed (non-portable) system, provide many reasons not to attempt to make such a processing system portable.
If one were thinking of changing a fixed processing system into a portable processing system, a natural approach would be to mount the system on wheels for ease of movement. However, landfill conditions make such an approach problematic. For example, a landfill typically provides variable, uneven terrain, which may include wet clay and spongy municipal solid waste. In wet conditions, the terrain is conducive to mud. Such conditions may make it difficult if not impossible to wheel conveyors or other equipment, and outriggers may be needed to stabilize the equipment.
Large capacity solid storage systems are also easily provided with a fixed system (e.g., using a silo), which is problematic if the system is to be made portable. Such large capacity, tall silos, for example, cannot easily be secured against wind loads, and are difficult to move. For example, if a system were to be made portable, it must be typically capable of processing 1-2 truckloads (25 tons/load) very rapidly (about 15 minutes/load, while the truck waits) without using tall vertical silos as used in fixed systems, as such silos cannot be safely transported in landfill conditions. By comparison, a fixed system with tall vertical silo(s) may take about 45 minutes to pneumatically load (per truck) and then another 15 minutes/truck to feed the pugmill and process the material.
A test for determining whether a liquid waste solidification process produces waste which can be legally entombed within a landfill is known as the Paint Filter Test (“the PFT”). The PFT is used to simulate the behavior of liquids contained in sorbents to be placed in landfills. The purpose of the PFT is to determine if excess liquids will be released from containerized wastes. The basis for the test derives from an EPA rule (Section 3004 (c) (2) of HSW A), which prohibits the placement of bulk or containerized liquids in landfills. The PFT is a simple, easy to conduct test that is relatively inexpensive to perform, and that has been used to determine the presence of free liquids in bulk or containerized waste since 1985. It consists of placing a sample (normally 100 ml or 100 g) into a conical paint filter (mesh number 60). The paint filter is suspended from a tripod or ringstand for five minutes. If any portion of the material passes through and drops from the filter in this time, the material is deemed to contain free liquids and cannot be disposed of in a landfill.
Another feature that can render material unsuitable for a landfill is dust. Various regulations monitor the amount of dust associated with landfill materials, given the danger to landfill operators and personnel in the vicinity of a landfill due to dust. It has been determined that dust regulations will be satisfied by a relatively simple test: the material to be placed in a landfill must be in the range of about at least 5% (20:1 solids-to-liquid ratio by weight)-20% (4:1 solids-to-liquid ratio, by weight) moisture by weight. Typically, this “moisture test” is determined by a laboratory, which weighs the material, then bakes off the moisture and weighs it again. However, an experienced landfill operator can typically determine whether the moisture test will be met by visual observation, coupled with periodic lab tests. If dust becomes an issue, the operator can simply add liquid to the pugmill.
As mentioned, vertical silos, which can hold 30 tons or more, and may be 35-feet or taller and 10-feet wide or more, present issues, including susceptibility to tipping due to differential settlement, seismic and/or wind loads. Additionally, due to landfill conditions of spongy, variable bearing material (with air gaps, voids, compressible in some places, and stone/not compressible in others), it can be both difficult to secure the base frame of the silo, or other enclosure and/or pressurized vessel, to the landfill surface, and also difficult to maintain the silo in a stable, level condition.
Additional problems can arise as material to be landfilled flows down through the vertical silo. As the material flows, such as through an upper cylindrical portion and then down through a narrowed conical portion, the material may enter a relatively narrow funnel area, where a slide gate sits, within a narrow (e.g., ⅛-inch clearance) slide gate track. The purpose of the slide gate is to selectively enter the funnel area through the slide gate track, and stop the downward flow of material, in case of a malfunction in the equipment; otherwise, the entire area can quickly fill with material from the upper silo portion, and “digging out” from literally tons of this material can result in substantial downtime. Currently, the slide gate is laboriously driven through the material present within the slide gate track, using a time-consuming, manual labor process in which sledge hammers are used to drive the slide gate into and through the slide track, necessitating crushing/compressing the material within the opening surrounded by the slide track, and also clearing the material away from the track edges and toward the opening, so that the slide gate can be moved within the slide gate track to cover the opening and to stop the flow of material within the funnel portion of the silo.
Currently, trucks unload landfill material into a feed hopper, which may have a conveyor in its bottom to allow this material to be conveyed directly to a pugmill, for example. Presently, there are no known commercial feed hoppers which allow a truck to fully unload into the feed hopper with insubstantial spillage. To “fully unload” requires the rear of the truck to actually fit inside the feed hopper's open front portion. Commercially available feed hoppers have been too narrow in width to allow this. Unless the truck can fully unload within a feed hopper, substantial spillage of material may result.